Tetrafunctional initiator

ABSTRACT

The melt of polyvinyl aromatic polymers comprising from 10 to 45 weight % of star branched polymer prepared using a combination of thermal and tetra functional peroxide initiation has an improved melt strength permitting better foam formation for extrusion foam blown with conventional blowing agents and inert gases including CO 2 .

FIELD OF THE INVENTION

[0001] The present invention relates to polymeric foams. Moreparticularly the present invention relates to foams of vinyl aromaticpolymers that comprise from 10 to 45 weight % of a star branched vinylaromatic polymer.

BACKGROUND OF THE INVENTION

[0002] In the manufacture of extrusion foam there are competing factorsto balance. One needs to consider the viscosity or melt flow rate of thepolymer as it impacts on the extruder output and the melt strength ofthe polymer, and particularly of the foaming mass as it leaves theextruder as this impacts on the bubble stability or the foam stability.If one makes a very low viscosity polymer it will flow through theextruder easily. However a low viscosity polymer tends to have a lowmelt strength and the resulting foam tends to have a lower stability.Accordingly, there is a tendency for foams of low viscosity to collapseupon extrusion or shortly after leaving the extruder.

[0003] It has been known for some time that the melt strength of apolymer may be improved by lightly cross linking the polymer. The paper“Some Effects of Crosslinking Upon the Foaming Behavior of HeatPlastified Polystyrene”, L. C. Rubens Journal of Cellular Plastics,April 1965, 311-320 discloses that polystyrene, containing small amounts(about 0.03 weight %) of divinyl benzene, may be foamed with CO₂ and thepolymer has good foam stability and good foam volume. This technology isalso the subject matter of U.S. Pat. Nos. 2,848,427 and 2,848,428 issuedAug. 19, 1958 to Louis C. Rubens assigned to The Dow Chemical Company.The technology comprised forming a cross linked polystyrene polymer thenimpregnating it in solid state with CO₂ then releasing the pressure andletting the polymer expand. This technology was not strongly relevant toextrusion foam techniques.

[0004] The cross linking technology was further applied in U.S. Pat. No.3,960,784 issued Jun. 1, 1976 to Louis C. Rubens assigned to The DowChemical Company. This patent teaches concurrent impregnation of apolymer with a blowing agent and a cross linking agent. The polystyreneis prepared at temperatures from about 60° C. to 120° C. preferably fromabout 70° C. to 100° C. (Column 3 lines 25-26). These temperature rangesare indicative of suspension polymerization and concurrent or postpolymerization impregnation with the blowing agent and cross linkingagent (see Example 3) although the polymer could be molded into thinsheets for the impregnation step. This reference does not teach anextrusion foam.

[0005] While divinyl benzene is useful in suspension polymerization ittends to produce gels in bulk or solution polymerization. In a bulk orsolution polymerization the use of tetra functional initiatorssignificantly reduces gels. Typically no or very low levels (e.g. lessthan 0.5 weight %, more generally less than 0.1 weight %) of gels (i.e.insoluble polymer in typical solvents).

[0006] With the Montreal protocol on reducing the use of CFC's andHCFC's and regulations regarding the permissible discharge of volatileorganic compounds (VOC's) there was increase pressure on polymer foamindustry to move to other blowing agents such as CO₂ or N₂.Representative of this type of art is Monsanto's Australian Patent529339 allowed Mar. 17, 1983 The patent teaches the formation of a foamby extruding polystyrene and injecting CO₂ into the extruder.Interestingly there is no mention of cross linking agents or branchedpolystyrene in the patent. U.S. Pat. No. 5,250,577 issued Oct. 5, 1993to Gary C. Welsh is similar as it pertains to extrusion foamingpolystyrene in an extrusion process using CO₂ as the sole blowing agent.Again there is no reference in U.S. Pat. No. 5,250,577 to the use ofcross linking agents.

[0007] At about this time U.S. Pat. No. 5,266,602 issued to Walter etal. assigned to BASF. The patent teaches foaming a branched polystyrene.The foaming agent is conventional (e.g. C₄₋₆ alkanes). The polymer isprepared in the presence of a peroxide initiator other than a benzoylcompound and a moderator (chain transfer agent) such as a mercaptan(e.g. t-dodecyl mercaptan) and a “branching agent”. The branching agentcontains a second unsaturation as a point for the polymer to branch.Suitable agents include divinyl benzene, butadiene and isoprene. Thesetypes of branching agents would not produce the star branched polymersreferred to herein. The actual polymerization process appears to be asuspension process. Additionally there is no reference in the disclosureto blowing the polystyrene with anything other than conventional alkaneblowing agents.

[0008] U.S. Pat. No. 5,576,094 issued Nov. 19, 1996 to Callens et al.assigned to BASF teaches extruding slab foamed polystyrene blown withCO₂ or a mixture of CO₂ and C₁₋₆ alcohols or ethers of C₁₋₄ alkyl alkoxycompounds. The polystyrene is a branched polystyrene preferably havingat least 50%, more preferably 60% of the polymer being a star branchedstyrene butadiene block polymer. The polymer has a VICAT softeningtemperature not greater than 100° C. This teaches against the subjectmatter of the present invention. Additionally the polymer has a meltindex MVI 200/5 of at least 5 mL10 minutes.

[0009] U.S. Pat. No. 5,830,924 issued Nov. 3, 1998 to Suh et al.assigned to The Dow Chemical Company claims a process for extruding aclosed cell foam using CO₂ or a mixture of CO₂, conventional alkaneblowing agents and a polystyrene in which from 50 to 100 weight % of thepolystyrene is star branched (i.e. branched). This teaches away from thesubject matter of the present invention which requires a different typeof polymer and lower weight % of star branched vinyl aromatic polymer.

[0010] U.S. Pat. No. 5,760,149 issued Jun. 2, 1998 to Sanchez et al.discloses tetrafunctional (monoperoxycarbonate) compounds that areuseful as initiators for olefin monomers including styrene. The patentalso teaches a process for polymerizing polystyrene. However, there isno teaching in the patent of foaming the resulting polymer usingextrusion techniques.

[0011] The present invention seeks to provide a novel process forextrusion foaming of styrenic polymers in which the styrenic polymercomprises less than 50 weight % of branched styrenic polymer.

SUMMARY OF THE INVENTION

[0012] The present invention provides a closed cell foam comprising fromC₈₋₁₂ vinyl aromatic polymer comprising:

[0013] i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromaticmonomers; and

[0014] ii) from 0 to 40 weight % of one or more monomers selected fromthe group consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acidand acrylonitrile and methacrylonitrile;

[0015] Which polymer may be grafted onto or occluded within from 0 to 12weight % of one or more rubbery polymers selected from the groupconsisting of:

[0016] iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

[0017] iv) copolymers comprising from 60 to 85 weight % of one or moreC₄₋₅ conjugated diolefins and from 15 to 40 weight % of a monomerselected from the group consisting of acrylonitrile andmethacrylonitrile, said vinyl aromatic polymer comprising 10 to 45weight % of a star branched polymer and having a VICAT softeningtemperature not less than 100° C.

[0018] In a further embodiment the present invention provides a processfor preparing the above closed cell foam comprising injection into amolten mass of C₈₋₁₂ vinyl aromatic polymer comprising:

[0019] i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromaticmonomers; and

[0020] ii) from 0 to 40 weight % of one or more monomers selected fromthe group consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acidand acrylonitrile and methacrylonitrile;

[0021] Which polymers are grafted onto from 0 to 12 weight % of one ormore rubbery polymers selected from the group consisting of:

[0022] iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

[0023] iv) copolymers comprising from 60 to 85 weight % of one or moreC₄₋₅ conjugated diolefins and from 15 to 40 weight % of a monomerselected from the group consisting of acrylonitrile andmethacrylonitrile, said polymer comprising 10 to 45 weight % of a starbranched polymer and having a VICAT softening temperature not less than100° C.; at a temperature from 140 to 235° C. and a pressure from 1500to 3500 psi from 2 to 15 weight % of one or more blowing agents selectedfrom the group consisting of C₄₋₆ alkanes, CFCs, HCFCs, HFCs, CO₂ and N₂and maintaining said C₈₋₁₂ vinyl aromatic polymer in a molten state andthoroughly mixing said blowing agent with said polymer and extrudingsaid mixture of blowing agent and polymer.

[0024] The present invention also provides a process for polymerizing avinyl aromatic monomer comprising from 5 to 45 weight % of star branchedvinyl aromatic polymer, comprising feeding a mixture comprising:

[0025] i) from 60 to 100 weight % of one or more C₈₋₁₂ vinyl aromaticmonomers; and

[0026] ii) from 0 to 40 weight % of one or more monomers selected fromthe group consisting of C₁₋₄ alkyl esters of acrylic or methacrylc acidand acrylonitrile and methacrylonitrile;

[0027] Which polymer may be grafted onto or occluded within from 0 to 12weight % of one or more rubbery polymers selected from the groupconsisting of:

[0028] iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and

[0029] iv) copolymers comprising from 60 to 85 weight % of one or moreC₄₋₅ conjugated diolefins and from 15 to 40 weight % of a monomerselected from the group consisting of acrylonitrile andmethacrylonitrile, and from 0.01 to 0.1 weight % of a tetrafunctionalperoxide initiator of the formula:

[0030]  wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals; and R is a neopentyl group, in the absence of a cross linkingagent to a series of two or more continuous stirred tank reactors, toprovide a relatively low temperature initial reaction zone at atemperature from 100 to 130° C. and a relatively higher temperaturesubsequent reaction zone at a temperature from 130 to 160° C. andmaintaining a ratio of residence time in said relatively lowertemperature reaction zone to said relatively higher temperature reactionzone from 1:1 to 3:1 and recovering the resulting polymer.

BEST MODE

[0031] As used in this specification “star branched” polymer meanshaving multiple, preferably at least 3, most preferably 4, brancheseminating from a common node.

[0032] The styrenic polymers of the present invention may be co- orhomopolymers of C₈₋₁₂ vinyl aromatic monomers. Some vinyl aromaticmonomers may be selected from the group consisting of styrene, alphamethyl styrene and para methyl styrene. Preferably the vinyl aromaticmonomer is styrene.

[0033] The styrenic polymer may be a copolymer comprising from 60 to 100weight % of one or more C₈₋₁₂ vinyl aromatic monomers; and from 0 to 40weight % of one or more monomers selected from the group consisting ofC₁₋₄ alkyl esters of acrylic or methacrylc acid and acrylonitrile andmethacrylonitrile. Suitable esters of acrylic and methacrylic acidinclude methyl acrylate, ethyl acyrlate, butyl acrylate, methylmethacrylate, ethyl methacrylate and butyl methacrylate.

[0034] The polymers of the present invention may be rubber modified.That is the polymer may be grafted onto or occluded within from 0 to 12weight % of one or more rubbery polymers selected from the groupconsisting of:

[0035] i) co- and homopolymers of C₄₋₅ conjugated diolefins; and

[0036] ii) copolymers comprising from 60 to 85 weight % of one or moreC₄₋₅ conjugated diolefins and from 15 to 40 weight % of a monomerselected from the group consisting of acrylonitrile andmethacrylonitrile.

[0037] The rubbery polymer may be selected from a number of types ofpolymers. The rubbery polymer may comprise from 40 to 60, preferablyfrom 40 to 50 weight % of one or more C₈₋₁₂ vinyl aromatic monomerswhich are unsubstituted or substituted by a C₁₋₄ alkyl radical and from60 to 40, preferably from 60 to 50 weight % of one or more monomersselected from the group consisting of C₄₋₅ conjugated diolefins. Suchpolymers are known as the styrene butadiene rubbers (SBR). The rubbermay be prepared by a number of methods, preferably by emulsionpolymerization. This process is well known to those skilled in the artand described for example in Rubber Technology, Second Edition, editedby Maurice Morton, Robert E. Krieger Publishing Company Malabar,Florida, 1973, reprint 1981—sponsored by the Rubber Division of theAmerican Chemical Society.

[0038] The rubbery polymer may be a nitrile rubber comprising from 15 to40 weight % of one or more monomers selected from the group consistingof acrylonitrile and methacrylonitrile, preferably acrylonitrile, andfrom 85 to 60 weight % of one or more C₄₋₆ conjugated diolefins. Thepolymers may be prepared by a number of methods, preferably by emulsionpolymerization. This process is well known to those skilled in the artand described for example in the aforementioned reference.

[0039] The rubber may be a co- or homopolymer of one or more C₄₋₆conjugated diolefins such as butadiene (1,3-butadiene) or isoprene,preferably butadiene. The polybutadiene may have a molecular weight (Mw)from about 260,000 to 300,000, preferably from about 270,000 to 280,000.Polybutadiene has a steric configuration. The polymer may have a cisconfiguration ranging from about 50% up to 99%. Some commerciallypolymers have a cis content of about 55% such as TAKTENE® 550 (trademarkof Bayer AG) or DIENE® 55 (trademark of Firestone). Some commerciallyavailable butadiene has a cis configuration from about 60 to 80% such asFirestone's DIENE® 70. Some high cis-butadiene rubbers may have a cisconfiguration of 95% or greater, preferably greater than 98% (TAKTENE®1202).

[0040] If present, preferably the rubber is present in an amount fromabout 3 to 10 weight % based on the total composition fed to the reactor(i.e. monomers and rubber). Polybutadiene is a particularly usefulrubber.

[0041] The process for making HIPS is well known to those skilled in theart. The rubber is “dissolved” in the styrene monomer (actually therubber is infinitely swollen with the monomer). This results in twoco-continuous phases. The resulting “solution” is fed to a reactor andpolymerized typically under shear. When the degree of polymerization isabout equal to the weight % of rubber in the system it inverts (e.g. thestyrene/styrene polymer phase becomes continuous and the rubber phasebecomes discontinuous. After phase inversion the polymer is finished ina manner essentially similar to that for finishing polystyrene.

[0042] The polymer is prepared using conventional bulk, solution, orsuspension polymerization techniques. However, there is added to thefirst reactor (i.e. the lower temperature reactor) from about 0.01 to0.1 weight % (100 to 1000 ppm) of a tetrafunctional peroxide initiatorof the formula:

[0043] Wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group. The reaction is conducted in theabsence of a cross linking agent. Preferably the tetrafunctionalperoxide is present in the feed to the first reactor (i.e. the lowertemperature reactor) in an amount from about 200 to 400 ppm (0.02 to0.04 weight %), most preferably from 250 to 350 ppm (0.025 to 0.035weight %).

[0044] Suitable tetrafunctional peroxide initiators include initiatorsselected from the group consisting oftetrakis-(t-amylperoxycarbonyloxymethyl) methane,tetrakis-(t-butylperoxycarbonyloxymethyl) methane, 1,2,3,4-tetrakis(t-amylperoxycarbonyloxy) butane and the tetrakis (t-C₄₋₆ alkylmonoperoxycarbonates). A particularly useful initiator is the compoundof the above formula wherein R is a nenopentyl group and R¹ is atertiary amyl or tertiary butyl radical.

[0045] Typically in a bulk or solution process the monomer mixture andoptionally rubber is polymerized in at least two continuous stirred tankreactors. The first reaction temperature is kept at a relatively lowtemperature from about 100 to 130° C., preferably from 120 to 130° C.and then at a relatively higher temperature from about 130 to 160° C.,preferably from about 135 to 145° C. In the polymerization process thereare competing initiation reactions. The initiation may be thermalwithout the use of the initiator or it may be initiated by the peroxycarbonate initiator. The residence time in each temperature zone iscontrolled so that the amount of polymerization initiated thermally(which results in a linear polymer) and by the peroxy carbonateinitiator (in which about half of the resulting polymer is branched) iscontrolled so that not more than 50 weight % of the resulting polymer isbranched. For example if the reaction is controlled so that the ratio ofresidence time at the lower temperature to time at higher temperature isfrom 1:1 to 3:1, preferably from 1.5:1 to 2.5:1, most preferably about2:1 (i.e. 1.8:1 to 2.2:1). The weight ratio of linear to star branchedpolymer is controlled to greater than 1:1 (e.g. greater than 50:50).Preferably, the vinyl aromatic polymer or styrenic polymer will comprisefrom about 10 to 45, preferably from about 15 to 40, most preferablyfrom about 15 to 30 weight % of a star branched polymer.

[0046] In a suspension process the monomers, optionally includingdissolved rubber, may be either first partially polymerized in acontinuously stirred tank system. The partially polymerized monomermixture has stabilizers or suspending agents added to it to help suspendit in the aqueous phase as an oil in water suspension. Typically thestabilizer or suspending agent is added in an amount from 0.1 to 2.0weight %, preferably from 0.5 to 1.0 weight %.

[0047] Useful stabilizers or suspending agents are well known to thoseskilled in the art. Useful stabilizers or suspending agents includepolyvinyl alcohol, gelatin, polyethylene glycol, hydroxyethyl cellulose,carboxymethyl cellulose, polyvinyl pyrrolidone, polyacrylamides, saltsof poly (meth) acrylic acid, salts of phosphonic acids, salts ofphosphoric acid and salts of complexing agents such as ethylene diaminetetraacetic acid (EDTA).

[0048] Generally the salts are ammonium, alkali and alkaline earth metalsalts of the foregoing stabilizers or suspending agents. For exampletricalcium phosphate is a suitable suspending agent.

[0049] The tetra functional initiator may be added to the monomermixture prior to polymerization in the bulk or mass reactor or justprior to suspension batch polymerization in the suspension batchreactor. The suspension batch reactor is generally operated at lowertemperatures than the bulk reactor. However, the suspension batchreaction is finished at higher temperatures from about 120 to 150° C.,typically from about 125 to 135° C.

[0050] The resulting polymer has a number of unique properties that makeit suitable for extrusion foaming and particularly suitable forextrusion foaming using inorganic blowing agents such as CO₂ or N₂. Thepolymer has a VICAT softening temperature (as measured by DIN 53460 isequivalent to ISO 306 is equivalent to ASTM D 1525-96) of greater than100° C., preferably from 105° C. to 115° C. The polymer has a mean meltstrength at 210° C. of not less than 12.5 cN.

[0051] The melt strength and the stretch ratio test are determined usinga Rosand® Capillary Rheometer. The mean melt strength is determined byextrusion of a melt at 210° C. of the polymer through a circular 2-mmdiameter flat die, length to diameter (L/D) of the die is 20:1. Thestrand is extruded at a constant shear rate of 20 sec⁻¹. The strand isattached to a haul off unit which increases in speed with time. Thestrand is attached to a digital balance scale to measure the force ofdraw on the polymer. As the speed of the haul off unit increases thedraw force increases. As a result the strand breaks. The draw forceimmediately prior to break is defined as the melt strength. The stretchratio is defined as the ratio of the velocity of draw to the extrusionvelocity at the die exit. The test is repeated at least three times todetermine an average value.

[0052] The polymer may have a melt flow at condition G (230° C./5 Kgload of less than 5 g/10 minutes, preferably less than 3 g/10 minutes,most preferably of less than 2 g/10 minutes. Additionally, the polymerhas a Mz which exceeds typical high heat crystal polystyrene resins byat least 40,000, preferably by greater than 60,000.

[0053] The polymer may be foamed using conventional extrusion foamingequipment. The extruder may be a back to back type or it may be amultizoned extruder having at least a first or primary zone to melt thepolymer and inject blowing agent and a second extruder or zone. In theprimary extruder or zone the polymer melt is maintained at temperaturesfrom about 425° F. to 450° F. (about 218 to 232° C.). Once the polymeris melted, blowing agent is injected into the melt at the end of theprimary extruder or zone. In the primary extruder or zone there will bea high shear zone to promote thorough mixing of the blowing agent withthe polymer melt. Such a zone may comprise a number of pin mixers.

[0054] The polymer melt containing dissolved or dispersed blowing agentis then fed from the primary extruder to the secondary extruder orpasses from a primary zone to a secondary zone within the extrudermaintained at a melt temperature of 269° F. to 290° F. (about 132° C. to143° C.). In the secondary extruder or zone the polymer melt andentrained blowing agent passes through the extruder barrel by the actionof an auger screw having deep flights and exerting low shear upon thepolymer melt. The polymer melt is cooled by means of cooling fluid,typically oil which circulates around the barrel of the extruder.Generally the melt is cooled to a temperature of from about 250° F. toabout 290° F. (about 121° C. to 143° C.).

[0055] The blowing agent may be selected from the group consisting ofC₄₋₆ alkanes, CFCs, HFCs, HCFCs, CO₂, N₂, air and mixtures thereof. Theblowing agent may be CO₂ per se or N₂ per se. The blowing agent maycomprise from 20 to 95 weight % of a blowing agent selected from thegroup consisting of one or more C₄₋₆ alkanes (as described below) andfrom 80 to 5 weight % of CFCs, HFCs and HCFC's (as described below).Suitable C₄₋₆ alkanes include butane, pentane and mixtures thereof.

[0056] The blowing agent may comprise from 30 to 95, preferably from 70to 95, most preferably from 80 to 90 weight % of CO₂ and from 70 to 5,preferably from 30 to 5, most preferably from 20 to 10 weight % of oneor more compounds selected from the group consisting of C₁₋₂ halogenatedalkanes and C₄₋₆ alkanes. Suitable C₁₋₂ halogenated alkanes include thechloroflurocarbons (CFCs); hydrofluorocarbons (HFCs) and thehydrochlorofluorocarbons (HCFCs) such as trichlorofluoromethane(CFC-11); dichlorodifluoromethane (CFC-12); trichlorotrifluoroethane(CFC-113); dichlorotetrafluoroethane (CFC-114); dichlorofluoromethane(CFC-21); chlorodifluoromethane (HCFC-22); difluoromethane (HFC-32);2-chloro-1,1,1,2-tetrafluoroethane (HCFC-1 24); pentafluoroethane(HFC-125); 1,1,1 ,2-tetrafluoroethane (HCFC-1 24);1,1-dichloro-1-fluoroethane (HCFC-141b); 1-chloro-1,1-difluoroethane(HCFC-142b); trifluoroethane (HFC-143a); 1,1-difluoroethane (HFC-152a);tetrafluoroethane (HFC-134a); and dichloromethane. However, due toenvironmental concerns it is preferred to use alkanes such as C₄₋₆alkanes which have not been halogenated such as butane, pentane,isopentane and hexane. The blowing agent system may be used in amountsfrom 2 to 15, preferably from 2 to 10, most preferably from about 3 to 8weight % based on the weight of the polymer.

[0057] The pressure within the extruder should be sufficient to keep theblowing agent in the polymer melt. Typically, the pressures in the meltafter the blowing system has been injected will be from about 1500 to3500 psi, preferably from about 2000 to about 2500 for CO₂. The CO₂ andthe other blowing agent may be injected separately into the melt. Ifthis is done, preferably the alkane and/or halogenated alkane will beinjected upstream of the CO₂ as these types of blowing agents have aplasticizing effect on the polymer melt which may help the CO₂ go intothe melt. The alkane blowing agent and the CO₂ may also be mixed priorto injection into the extruder as is disclosed in U.S. Pat. No.4,424,287 issued Jan. 3, 1984 assigned to Mobil Oil Corporation.

[0058] To improve the cell size and/or distribution throughout thepolymer small amounts of a nucleating agent may be incorporated into thepolymer blend or solution. These agents may be physical agents such astalc or they may be agents which release small amounts of CO₂ such ascitric acid and alkali or alkaline earth metal salts thereof and alkalior alkaline earth metal carbonates or bicarbonates. Such agents may beused in amounts from about 500 to 5,000 ppm, typically from about 500 to2,500 ppm based on the polymer melt or blend.

[0059] The polymer melt or blend may also contain the conventionaladditives such as heat and light stabilizers (e.g. hindered phenols andphosphite or phosphonite stabilizers) typically in amounts of less thanabout 2 weight % based on the polymer blend or solution.

[0060] The foam is generally extruded at atmospheric pressure and as aresult of the pressure release on the melt, the melt foams. The foam iscooled to ambient temperature typically below about 25° C., which isbelow the glass transition temperature of the polymer and the foam isstabilized. One of the advantages of the present invention is that thefoamed polymer melt has better melt strength than the foamed polymermelts of the prior art and there is less foam collapse.

[0061] The foam may be extruded onto rollers as a relatively thick slabtypically from about 1 to 3 inches thick. The foam density may vary from2 to 15 lbs/ft³ (from about 0.03 to 0.24 g/cm³). The slab is cut intoappropriate lengths (8 feet) and is generally used in the constructionindustry. Thinner foams, typically from about {fraction (1/16)} to about¼ inches (62 to 250 mils) thick may be extruded as slabs or as thinwalled tubes which are expanded and oriented over an expanding tubularmandrel to produce a foam tube which is slit to produce sheet. Theserelatively thin sheets are aged, typically 3 or 4 days and then may bethermoformed into items such as coffee cups, meat trays or “clamshells”.

[0062] The present invention will now be illustrated by the followingnon-limiting examples in which, unless otherwise indicated parts meansparts by weight (grams) and percent means weight percent.

EXAMPLE 1 Polymer Preparation

[0063] Styrene monomer and 0.028 weight % of a tetra t-alkylperoxycarbonate sold by Ato Chemie under the trade mark JWEB50 were first fedinto a continuously stirred tank reactor maintained at 120° C. Theresidence time in the first reactor was about 2.5 hours. The partiallypolymerized mixture from the first reactor was then fed to a secondcontinuously stirred tank reactor maintained at 140° C. The residencetime in the second reactor was about 1 hour. The resulting polymer wasthen devolatilized in a falling strand devolatilizer and recovered andpelletized.

[0064] The reaction conditions were such that about 64% of the polymerwas thermally initiated and linear. About 36% of the polymer wasinitiated by the peroxide and about half of the resulting polymer wasstar branched. The polymer had an Mz from 40,000 to 75,000 greater thanconventional high heat crystal.

EXAMPLE 2

[0065] The procedure of Example 1 was repeated except that the amount ofinitiator was 0.045 weight %.

EXAMPLE 3

[0066] The procedure of Example 1 was repeated except that zinc stearatewas also included in the polymer in an amount of about 0.1 weight %.

[0067] Physical Properties

[0068] The physical properties of the resins prepared in Examples 1, 2and 3 were compared to NOVA Chemicals' VEREX™ 1280 polystyrene resin and1230 polystyrene resin (both linear crystal polystyrene resins used inextrusion foam applications), and Dow's resin 685D (a branchedpolystyrene resin). The results are set forth in Table 1.

EXAMPLE 4

[0069] The above samples together with the reference samples wereextrusion foamed using pentane as the blowing agent. The average celldiameter of the foam was measured. The results are set out in Table 2.

[0070] The foams extruded well and the cell data suggests that the foamstability good. The resulting foams have good toughness. TABLE 1Polystyrene Sample Identification Example 1 Example 2 Example 3Initiator: polyether tetrakis 280 ppm 450 ppm 280 ppm VEREX 1280 VEREX1230 Dow 685D Initiator Initiator Initiator + Zn (t-butylperoxycarbonate) Mw 351,000 345,000 342,000 306,000 309,000 310,000 Mn 132,000113,000 141,000 77,000 102,000 130,000 Mz 638,000 659,000 606,000535,000 551,000 550,000 Polydispersity (Mw/Mn) 2.66 3.05 2.42 3.97 3.032.38 Mean Melt Strength (cN) @ 190° C. 38.21 36.95 37.44 31.07 30.4234.7 Mean Stretch Ratio (%) @ 190° C. 91.8 81 79.9 84.3 99.4 91.8 MeanPeak Melt Strength @ 190° C. 45.25 45.73 43.7 38.74 34.75 39.73 MeanMelt Strength (cN) @ 210° C. 14.11 14 14.56 10.25 11.02 11.95 MeanStretch Ratio (%) @ 210° C. 279.6 230.3 236.5 428.6 399.4 326 Mean PeakMelt Strength @ 210° C. 17.43 17.02 17.73 12.22 12.87 14.43 Notched Izod(ft-lb/in) 0.36 0.32 0.34 0.33 0.32 0.22 Melt Flow Condition “G” (g/10min) 1.35 1.74 1.4 1.98 2.07 1.42 VICAT (° C.) 108.4 109 108.9 108.2108.6 109.9

[0071] TABLE 2 Polystyrene Sample Identification Example 1 Example 2Example 3 Initiator: polyether tetrakis 280 ppm 450 ppm 280 ppm Verex1280 Verex 1230 Dow 685D Initiator Initiator Initiator + Zn(t-butylperoxy carbonate) Isopentane fed to foam process (wt %) 5 5 5 55 5 Test Results on 2S Type Foamed Meat Trays Molded From PolystyreneSamples Mean Load at Max Load (lbs) 2.44 2.56 2.84 3.2 2.88 2.96 MeanDisplacement at Max Load (in.) 2.16 1.92 1.46 1.71 2.13 2.28 Mean Loadat 1.5″ Deflection (lbs) 2.2 2.44 2.82 3.16 2.67 2.62 Mean Slope(lbs/in) 3.02 3.86 3.58 3.74 3.61 3.47 Mean Part Weight (grams) 4.5214.595 4.59 4.96 4.758 4.93 Mean Sidewall Thickness (inches) 0.091 0.0870.105 0.102 0.103 0.098 Mean Foam Density (lbs/ft³) 3.2 3.56 2.92 3.073.02 3.194 Mean Orientation MD (%) 55.93 57.48 56.07 54.4 56.68 54.49Mean Orientation TD (%) 57.15 57.2 56.55 54.73 56.47 52.29 Number ofCells Across Sheet Thickness (TD) 21 20 38 22 31 23 Average CellDiameter (mm) 0.1101 0.1105 0.0702 0.1178 0.0844 0.1082 Number of PartsWith Sidewall Cracks 0 1 0 3 0 2 Cell Structure coarse fine coarsecoarse Cell Shape slight slight spherical spherical spherical sphericalelongation elongation Corner Inversion Test on Trays - Failure 0 3 0 2 06 Rate/20

What is claimed is:
 1. A closed cell foam comprising from C₈₋₁₂ vinylaromatic polymer comprising: i) from 60 to 100 weight % of one or moreC₈₋₁₂ vinyl aromatic monomers; and ii) from 0 to 40 weight % of one ormore monomers selected from the group consisting of C₁₋₄ alkyl esters ofacrylic or methacrylc acid and acrylonitrile and methacrylonitrile;Which polymer may be grafted onto or occluded within from 0 to 12 weight% of one or more rubbery polymers selected from the group consisting of:iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and iv)copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidvinyl aromatic polymer comprising 10 to 45 weight % of a star branchedpolymer and having a VICAT softening temperature not less than 100° C.2. The closed cell foam according to claim 1, wherein the star branchedvinyl aromatic polymer is present in an amount from 15 to 40 weight % ofthe vinyl aromatic polymer.
 3. The closed cell foam according to claim2, having a mean melt strength at 210° C. of not less than 12.5 cN. 4.The closed cell foam according to claim 2, wherein the vinyl aromaticpolymer has a VICAT softening temperature from 105 to 115° C.
 5. Theclosed cell foam according to claim 4, wherein the C₈₋₁₂ vinyl aromaticmonomer is selected from the group consisting of styrene, alpha methylstyrene and para methyl styrene.
 6. The closed cell foam according toclaim 5, wherein the vinyl aromatic polymer is prepared by solution orbulk polymerization in the presence of from 0.01 to 0.1 weight % of atetra functional peroxide initiator of the formula:

Wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent.
 7. The closed cell foam according to claim 4, wherein the C₈₋₁₂vinyl aromatic monomer is selected from the group consisting of styrene,alpha methyl styrene and para methyl styrene.
 8. The closed cell foamaccording to claim 7, wherein the C₈₋₁₂ vinyl aromatic polymer is ahomopolymer.
 9. The closed cell foam according to claim 8, wherein theC₈₋₁₂ vinyl aromatic polymer is polystyrene.
 10. The closed cell foamaccording to claim 9, wherein the tetrafunctional initiator is selectedfrom the group consisting of tetrakis-(t-amylperoxycarbonyloxymethyl)methane, and tetrakis-(t-butylperoxycarbonyloxymethyl) methane.
 11. Theclosed cell foam according to claim 10, having a melt flow at conditionG of less than 2.5 g/10 minutes.
 12. The closed cell foam according toclaim 11, which contains no rubbery polymer.
 13. The closed cell foamaccording to claim 11, wherein the rubbery polymer is present in anamount from 3 to 10 weight %.
 14. A process for preparing a closed cellfoam according to claim 1, comprising injection into a molten mass ofC₈₋₁₂ vinyl aromatic polymer comprising: i) from 60 to 100 weight % ofone or more C₈₋₁₂ vinyl aromatic monomers; and ii) from 0 to 40 weight %of one or more monomers selected from the group consisting of C₁₋₄ alkylesters of acrylic or methacrylc acid and acrylonitrile andmethacrylonitrile; Which polymers are grafted onto from 0 to 12 weight %of one or more rubbery polymers selected from the group consisting of:iii) co- and homopolymers of C₄₋₅ conjugated diolefins; and iv)copolymers comprising from 60 to 85 weight % of one or more C₄₋₅conjugated diolefins and from 15 to 40 weight % of a monomer selectedfrom the group consisting of acrylonitrile and methacrylonitrile, saidpolymer comprising 10 to 45 weight % of a star branched polymer andhaving a VICAT softening temperature not less than 100° C.; at atemperature from 140 to 235° C. and a pressure from 1500 to 3500 psifrom 2 to 15 weight % of one or more blowing agents selected from thegroup consisting of C₄₋₆ alkanes, CFCs, HCFCs, HFCs, CO₂, N₂ andmixtures thereof and maintaining said C₈₋₁₂ vinyl aromatic polymer in amolten state and thoroughly mixing said blowing agent with said polymerand extruding said mixture of blowing agent and polymer.
 15. The processaccording to clam 14, wherein the star branched vinyl aromatic polymeris present in an amount from 15 to 40 weight % of the vinyl aromaticpolymer.
 16. The process according to claim 15, wherein the vinylaromatic polymer has a mean melt strength at 210° C. of not less than12.5 cN.
 17. The process according to claim 16, wherein the vinylaromatic polymer has a VICAT softening temperature from 105 to 115° C.18. The process according to claim 17, wherein the C₈₋₁₂ vinyl aromaticmonomer is selected from the group consisting of styrene, alpha methylstyrene and para methyl styrene.
 19. The process according to claim 18,wherein the vinyl aromatic polymer is prepared by solution or bulkpolymerization in the presence of in the presence of from 0.01 to 0.1weight % of a tetra functional peroxide initiator of the formula:

Wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent.
 20. The process according to claim 19, wherein the C₈₋₁₂ vinylaromatic monomer is selected from the group consisting of styrene, alphamethyl styrene and para methyl styrene.
 21. The process according toclaim 20, wherein the C₈₋₁₂ vinyl aromatic polymer is a homopolymer. 22.The process according to claim 21, wherein the C₈₋₁₂ vinyl aromaticpolymer is polystyrene.
 23. The process according to claim 22, whereinthe tetrafunctional initiator is selected from the group consisting oftetrakis-(t-amylperoxycarbonyloxymethyl) methane, andtetrakis-(t-butylperoxycarbonyloxymethyl) methane.
 24. The processaccording to claim 23, wherein the vinyl aromatic polymer has a meltflow at condition G of less than 2.5 g/10 minutes.
 25. The processaccording to claim 24, which contains no rubbery polymer.
 26. Theprocess according to claim 25, wherein the blowing agent is one or moreblowing agents selected from the group consisting of one or more C₄₋₆alkanes.
 27. The process according to claim 26, wherein the blowingagent is selected from the group consisting of pentane, butane andmixtures thereof.
 28. The process according to claim 25, wherein theblowing agent comprises from 20 to 95 weight % of a blowing agentselected from the group consisting of one or more C₄₋₆ alkanes and from80 to 5 weight % of CFCs, HFCs and HCFCs.
 29. The process according toclaim 28, wherein said one or more C₄₋₆ alkanes is selected from thegroup consisting of pentane, butane and mixtures thereof.
 30. Theprocess according to claim 25, wherein said blowing agent is CO₂. 31.The process according to claim 25, wherein the blowing agent comprisesfrom 30 to 95 weight % of CO₂ and from 70 to 5 weight % of one or moreblowing agent selected from the group consisting of C₄₋₆ alkanes andCFCs, HFCs and HCFCs.
 32. The process according to claim 31, whereinsaid one or more C₄₋₆ alkanes is selected from the group consisting ofpentane, butane and mixtures thereof.
 33. The process according to claim31, wherein the blowing agent comprises from 70 to 95 weight % of CO₂and from 30 to 5 weight % of one or more C₄₋₆ alkane blowing agentsselected from the group consisting of pentane, butane and mixturesthereof.
 34. The process according to claim 24, wherein the rubberypolymer is present in an amount from 3 to 10 weight %.
 35. The processaccording to claim 34, wherein the blowing agent is one or more blowingagents selected from the group consisting of one or more C₄₋₆ alkanes.36. The process according to claim 35, wherein the blowing agent isselected from the group consisting of pentane, butane and mixturesthereof.
 37. The process according to claim 34, wherein the blowingagent comprises from 20 to 95 weight % of a blowing agent selected fromthe group consisting of one or more C₄₋₆ alkanes and from 80 to 5 weight% of CFCs, HFCs and HCFCs.
 38. The process according to claim 37,wherein said one or more C₄₋₆ alkanes is selected from the groupconsisting of pentane, butane and mixtures thereof.
 39. The processaccording to claim 34, wherein the blowing agent comprises from 30 to 95weight % of CO₂ and from 70 to 5 weight % of one or more blowing agentselected from the group consisting of C₄₋₆ alkanes and CFCs, HFCs andHCFCs.
 40. The process according to claim 39, wherein said one or moreC₄₋₆ alkanes is selected from the group consisting of pentane, butaneand mixtures thereof.
 41. The process according to claim 39, wherein theblowing agent comprises from 70 to 95 weight % of CO₂ and from 30 to 5weight % of a C₄₋₆ alkane blowing agent selected from the groupconsisting of pentane, butane and mixtures thereof.
 42. The processaccording to claim 34, wherein the blowing agent is CO₂.
 43. A processfor polymerizing a vinyl aromatic monomer comprising from 5 to 45 weight% of star branched vinyl aromatic polymer comprising feeding a mixturecomprising: i) from 60 to 100 weight % of one or more C₈₋₁₂ vinylaromatic monomers; and ii) from 0 to 40 weight % of one or more monomersselected from the group consisting of C₁₋₄ alkyl esters of acrylic ormethacrylc acid and acrylonitrile and methacrylonitrile; Which polymermay be grafted onto or occluded within from 0 to 12 weight % of one ormore rubbery polymers selected from the group consisting of: iii) co-and homopolymers of C₄₋₅ conjugated diolefins; and iv) copolymerscomprising from 60 to 85 weight % of one or more C₄₋₅ conjugateddiolefins and from 15 to 40 weight % of a monomer selected from thegroup consisting of acrylonitrile and methacrylonitrile, and from 0.01to 0.1 weight % of a tetrafunctional peroxide initiator of the formula:

 wherein R¹ is selected from the group consisting of C₄₋₆ t-alkylradicals and R is a neopentyl group, in the absence of a cross linkingagent to a series of two or more continuous stirred tank reactors, toprovide a relatively low lo temperature initial reaction zone at atemperature from 100 to 130° C. and a relatively higher temperaturesubsequent reaction zone at a temperature from 130 to 160° C. andmaintaining a ratio of residence time in said relatively lowertemperature reaction zone to said relatively higher temperature reactionzone from 1:1 to 4:1 and recovering the resulting polymer.